Two stage electrohydraulic pressure control valves are increasingly being used in diverse applications for controlling such things as hydraulic control valves, brakes, clutches, and so forth. In many cases, those valves are controlled by a microprocessor and the control system, including such valves, can be made extremely versatile simply by changing the microprocessor program. One example is where a two stage electrohydraulic pressure control valve is used to controllably release and engage a spring applied pressure released brake. Such brakes are normally maintained in a disengaged position by a high electrical signal directed to the solenoid actuated pilot stage and is engaged by decreasing the electrical power to the solenoid. Under normal operating conditions, the electronics can be programmed to control the rate of decrease in the electrical power and thus modulate the engagement of the brake under controlled conditions.
However, one of the problems encountered occurs when an electrical malfunction occurs and the control valve is uncontrollably moved to a brake engaged position. When this happens, the brakes could be applied very abruptly without any warning to the operator.
Thus, what is needed is a two stage electrohydraulic pressure control valve that includes a mechanism for modulatably controlling the engagement of a spring applied pressure released brake when the electrical power to the solenoid is suddenly lost.
The present invention is directed to overcoming one or more of the problems as set forth above.